The original U.S. Pat. No. 5,152,945 was directed primarily to a means for and a method of forming a rock bolt of fiberglass reinforced plastics material (particularly a polyester formulation), but did not disclose any means for or method of rendering a rock bolt, electrically conductive. The parent specification discloses a method of rendering a rock bell electrically conductive, and disclosed a method of production of such a rock bolt. The method disclosed in that specification indicated the need to supply sufficient pressure to wetted tows to cause flow of resin into contact with bolt thread forming surfaces, and further disclosed applying chopped strand rainforced fluid but settable plastics mix to the wetted tows at the locality of a thread forming surface, before closure of dies. Further, the method disclosed forming a bolt with a shank, longer than the shank, forming surface of the dies by firstly forming a bolt with a shank equal to the length of the shank, forming surface, and withdrawing the threaded end outwardly from between the dies so as to form further wetted tows of a bundle between the shank forming cavities of the die end again closing the die to clamp portion of the shank end applying pressure to the wetted tows while the resin sets. Pressure was also indicated to be required to force resin to flow into close contact with thread forming surfaces, and protuberance recesses.
The parent specification disclosed (in FIG. 6) drawing a bundle of resin wetted fibers through an open die as a trailing extension of a premoulded product, and separating the moulded products externally of the die,
This invention relates to the abovementioned method, and to further enhancements of the production method disclosed in the parent specification. In particular, there is a requirement which is frequently met of producing an elongate product of a length which is within a tolerance specified by a customer, and further, quite often there is a requirement for a fiber reinforced plastics product which is suitable for use as a reinforcement bar for reinforcing of concrete. This is particularly desirable (when prestressed) because of the much lower Yourigs modulus of FRP compared with steel which is otherwise used, and the second favourable characteristic of FRP is that of having a very high yield point, yield taking place close to the ultimate tensile strength of the product if in tension, Still further, when embodied in concrete, the corrosion problem which is encountered by steel is not encountered by plastics. This in turn simplifies the structure of a concrete beam, for example, because there is not the need for a thick overlay of concrete to nearly such an extent as occurs is with metal, to avoid "concrete cancer", that is, corrosion of steel within the concrete, with increase of volume causing further cracking and corrosion.
In an aspect of this invention, withdrawal of a precured product from an open die is only partial, the moulded tail end functioning as a "plug" at the downstream end of the die so as to enable high die pressures to be used. The degree of withdrawal can be varied, so that products can be moulded to varying lengths which are greater than the die length and in the case of concrete reinforcing bar, to a tolerance of one pitch of the protuberances on its outer surface.
The invention can be extended to other products, which are not necessarily of constant cross-sectional shape throughout their lengths, in those instances, wherein pad of the product length has a constant cross-section, products can be made to a very small length tolerance.
In many applications of fiberglass reinforced plastics (FRP) although the tensile strength may be adequate because it is imparted by the unidirectional fibers, the torsional strength is inadequate, and it is an object of this invention to provide a method of production of an elongate fiber reinforced product which has a more reliable torsional strength than products presently produced by known compression methods. In an aspect of this invention, entrained air is substantially removed from the product.
In both the original specification and in the parent specification some pressure was indicated as being desirable during the moulding of a rock bolt, in order to cause a flow of resin through the fibers into cavities defined by the die surfaces, but nevertheless with the relatively small pressures which were contemplated at the time that specification was prepared, a consistent torsional strength was difficult to obtain. However, it has been found that by increasing the pressures to pressures greater than 200 psi (1400 kPa) for low viscosity resin, below 500 centipoise, the air inclusion which frequently occurs in products made from wetted tows, is eliminated, or greatly reduced, and what would otherwise be unwet fibers are fully wetted, so that there is no discontinuity of resin at that point, which otherwise occurs if very low pressures are used. Desirably the pressures exceed 500 psi (3500 kPa) for viscosities up to 2000 centipoise and 700 psi (5000 kPa) for viscosities above 5000 centipoise, or for densely packed fibers. Efforts have been made by the inventors to get even higher pressures. It is believed that under those pressures the air actually dissolves in the resin, but does not appear to have any deleterious effect on the shear strength of the resin which is relevant to the torsional strength of the product.
A problem which is encountered if high pressure is used in moulding a fiber reinforced product is that the resin tends to be extruded from the ends of a die through which the product extends. If that extrusion takes place at the upstream end of a hot die, a bulbous knob of set resin will form which needs to be removed, and that operation is not viable for production of continuous uniform length products.
With the object of inhibiting extrusion of resin through die ends and preventing a bulbous knob from forming at the entry end of the die, in an embodiment of this invention, use is made of clamping means comprised in the die, and having a cold surface at the upstream end of the die, the clamping means preferably being separate blocks appended to the die ends, while obviously they can be constituted by portion of the die itself at the upstream end, if it is kept water cooled.
In most applications of fiber reinforced tensioning members, bolts and reinforcing bar, tensile strength is the most desirable feature, and the tensile strength is largely a function of the amount of reinforcing fiber which exists in a given cross-sectional area in a product. For that reason, it is an object of this invention to provide improvements whereby the ratio of fibers to resin is much higher than what has been commercially produced previously, achieved by using the abovementioned higher pressures.